The invention generally relates to the field of starter motors and alternators, including starter motors and alternators for truck and automotive applications.
The Integrated Starter Alternator Damper (ISAD) is an electric machine used in the hybrid automotive and truck industries. As its name suggests, the ISAD integrates the functions of a starter, alternator, and flywheel damper, when connected to an engine. As indicated in FIG. 1, the ISAD electric machine 12 includes a rotor in electromagnetic communication with a stator 16 covered by a stator frame 17. The rotor 14 is a squirrel cage design made of cast aluminum and is designed for positioning within the stator 16. The stator 16 includes three phase windings 18 wound on the stator (i.e., the armature windings). The three phase windings 18 are connected to terminals 20 that extend from the stator frame 17. The terminals 20 are used to connect the three phase windings to an inverter/rectifier 22, located in an electronic control box.
FIG. 2 shows the arrangement of the ISAD rotor 14 and stator 16 in relation to an automotive engine 34. As shown in FIG. 2, the ISAD electrical machine is installed directly on the engine crankshaft 36 and linked to a gearbox 38 through a torque converter or clutch 40. The ISAD rotor 14 acts as a flywheel for the engine. The clutch disengages the gearbox from the ISAD during engine cranking, and connects the gearbox to the ISAD and the engine when power is required to the gearbox. Because the ISAD rotor is connected to the engine crankshaft, the speed of the electric machine has the same speed as the engine. However, the present invention is not limited to those arrangements where the electric machine has the same speed as the engine. For example the invention may also be used with integrated starter and alternator combinations where a belt drives the rotor of the electric machine at a different speed from the engine.
FIG. 3 shows a general overview of the electrical connections of the ISAD electric machine within the engine. The ISAD electric machine 12 shown in FIG. 3 can be any of a number of different three phase alternating current electric machines, including, but not limited to, permanent magnet electric machines, induction machines or synchronous machines. The windings of the electric machine 12 are connected to an inverter/rectifier 22. The voltage difference between each line connecting the armature windings to the inverter/rectifier is the AC line-to-line voltage, UAC. The inverter/rectifier 22 acts as an inverter to transform DC voltage into AC voltage when the electric machine operates as a motor, and acts as a rectifier to transform AC voltage into DC voltage when the electric machine operates as a generator. On the opposite side of the inverter/rectifier from the electric machine is a DC bus 24. The DC bus includes a battery (or batteries) 26 and capacitor 28 connected in parallel with the inverter/rectifier 22. A DC bus voltage, UDC, exists on the DC bus 24. A DC/DC converter 30 is connected to the DC bus 24 to step down any DC voltages (e.g., 36V DC to 12V DC), as required by the load. A DC/AC converter 32 is also connected to the DC bus to transform the DC voltage into AC voltage, or a reverse transformation, as required by the load.
Most ISAD systems strive to meet several design parameters. For example, during cranking it is preferable for cranking torque to continuously be as high as possible, and at least greater than breakaway torque (e.g., 250 Nm). This high torque is needed even at low temperatures such as −29° C., as may be experienced in the cold of winter. Also, to reduce pollution, it is desirable for the cranking speed to be near or equal to idle speed (e.g., 450˜500 rpm). During the short period of time when the ISAD is cranking the engine, the battery and capacitor, in parallel, provide the ISAD with the required cranking voltage through the DC/AC inverter 22. The maximum available electromagnetic torque at each speed can be used to crank the engine since the efficiency and power factor are not usually of significant concern during engine cranking. According to electric machine principles, the cranking torque is nearly proportional to the square value of the phase voltage of the machine (this especially true for induction machines). Therefore, sufficient phase voltage is an important issue because of its relation to cranking torque, and the efficiency and power factor are of particular relevance only if they affect the cranking voltage, and consequently the torque. Insufficient phase voltage to crank an engine is often encountered at low temperatures with batteries that are not fully charged. Accordingly, increased phase voltage is advantageous, as the cranking torque at all temperatures will increase if the phase voltage of the ISAD machine can be raised.
Once the engine is cranked, the electric power for charging batteries and running electric loads is required instantly. At this time, the ISAD rotor, which is installed directly on the crankshaft, settles in at its idle speed (e.g., 500 rpm). At idle speed the ISAD must provide at least a portion of the electric power required for normal operation of routine electric or electronic devices, such as air condition, heating system, electric blower, lights and audio or video system etc. Fifty-percent (50%) or more of the electric power required for normal operation is a typical design parameter.
After the engine launches and settles to idle, the ISAD operates as generator from idle speed through redline speed (e.g., 500˜7000 rpm). At a boosted engine speed (e.g., 1000 rpm), the machine has to supply full electric power to all electric loads, onboard and/or off-board, if applicable. The capacity of the full electric power output and the high efficiency should be maintained continuously from the boosted speed till redline speed (e.g., 7000 rpm).
The following equations are provided to further describe the design principles associated with an ISAD machine.
The back EMF per phase, produced by resultant air-gap flux, can be written as set forth in the following equation:Eg=√{square root over (2)}πfNφKdpΦ  (equation 1)where NΦ is winding turns per phase in series; Kdp is the winding factor including distribution, pitch and skewing factors; Φ is the resultant flux amplitude per pole; and f is the frequency of AC voltage.
The frequency, f, of AC voltage can be given by the following equation:                     f        =                  {                                                                                          p                    ⁢                                                                                   ⁢                    n                                    60                                                                              for                  ⁢                                                                           ⁢                  synchronous                  ⁢                                                                           ⁢                  machine                                                                                                                          p                    60                                    ⁢                                      n                                          1                      -                      s                                                                                                                    for                  ⁢                                                                           ⁢                  induction                  ⁢                                                                           ⁢                  machine                                                                                        (                  equation          ⁢                                           ⁢          2                )            where p is the number of pole pairs; s is the slip of induction machine whose value is positive for motoring state and negative for generating state; n is the speed of electric machine.
In the design of electrical machines, the relation between phase voltage and resultant back EMF can be given by the following equation:Uph=(1±ε)Eg  (equation 3)where ε is a percentage variation because of voltage drop cross leakage impedance of the armature windings, depending on the phase current I1, armature resistance R1 and leakage reactance X1, the “+” sign corresponds to motoring state, and the “−” sign corresponds to the generating state of the ISAD machine. That is, Uph>Eg for motoring operation, and Uph<Eg for generating operation. On the AC side of ISAD system in FIG. 3, the phase voltage Uph can be given by line-to-line voltage UAC.                               U                      p            ⁢                                                   ⁢            h                          =                  {                                                                      U                                      A                    ⁢                                                                                   ⁢                    C                                                                                                for                  ⁢                                                                           ⁢                  Δ                  ⁢                                      -                                    ⁢                  connection                                                                                                                          U                                          A                      ⁢                                                                                           ⁢                      C                                                                            3                                                                                                for                  ⁢                                                                           ⁢                  Y                  ⁢                                      -                                    ⁢                  connection                                                                                        (                  equation          ⁢                                           ⁢          4                )            
If a pulse width modulation frequency variable method is introduced to control the ISAD machine, the relation between AC line-to-line UAC and DC bus voltage UDC is governed byUAC(rms)=KCMaUDC  (equation 5)where Kc is conversion coefficient and its value is between 0.61 and 0.78 depending on control strategies, such as vector space control, six-step control etc.; Ma≦1 is the modulation depth for the PWM method. DC bus voltage can be written asUDC=Ubatteryμμ|IDC|Rbattery  (equation 6)where μ stands for non-constant battery resistance (which depends upon temperature and other factors), the “−” sign stands for the discharging state corresponding to motor operation, and the “+” sign stands for the charging state of batteries corresponding to generating state of the ISAD machine. In equation 6, the open circuit voltage Ubattery and internal Resistance Rbattery are not constants, depending on variables like temperature, charged state and even the current.
FIGS. 4(a) and 4(b) show an example of torque and voltage specifications and design requirements for a typical ISAD machine. In particular, FIG. 4(a) shows the design relationship between torque and speed for a typical ISAD machine, and FIG. 4(b) shows an example of the design relationship between phase voltage and speed in a typical ISAD machine.
With reference to FIG. 4(a), Tmax is the output torque available from the electric machine to crank the engine. This torque is delivered from the ISAD machine over the segment ATBT until the speed of the machine reaches ncrank and the engine fires. As discussed previously, it is preferable for cranking torque to be as high as possible. As torque requirements increase, a larger machine size is required to meet high cranking torque at severe conditions. Otherwise the system will face an insufficient cranking voltage challenge. With sufficient torque, the engine will crank and fire. Thereafter, the output torque of the electric machine falls off, as shown between points BT and CT. With reference to FIG. 4(b), the phase voltage of the electric machine continually increases with speed until the cranking speed is reached, as shown by segment AuBu.
After the engine fires, the electric machine acts as a generator as torque is input to the electric machine from the engine and the electric machine delivers electric current to the inverter/rectifier. Accordingly, the generated torque, TG, is shown in FIG. 4(a) as a negative value. Once the ISAD electric machine starts operating as a generator, it would be ideal for the electric machine to immediately output maximum electric power capable of satisfying the electrical requirements for the entire load (i.e., 100% loading). This 100% loading level is shown by segment EUFU in FIG. 4(b). However, if the ISAD electric machine were designed to output maximum electric power at the idle speed of the engine (i.e., point EU in FIG. 4(b)), a dramatic increase in the size of the electric machine would be required, since this would be equivalent to an electric machine design with a large torque at low speed (i.e., point ET in FIG. 4(a)). On the other hand, if the electric machine is only required to provide a part of the necessary electric power (e.g., 50% fill load) at idle speed, as shown by point DU in FIG. 4(b), and the full electric power output from the electric machine is delayed until the electric machine is operating at a boosted speed, the machine size can be reduced since the design point is shifted to DT in FIG. 4(a). In this situation, because the electric machine is only providing a part of the full electric power, some loads may be turned off until the machine reaches a boosted speed. This method is known as the “load match” method and is widely used for design of alternators for heavy duty vehicles with external electric loads. Of course, when the electric machine is only providing a part of the full electric power requirement at idle speed, the battery is not being charged. In fact, sometimes the battery may actually be picking up part of the load requirement. Therefore, it would be preferable for the electric machine to provide as much output power as possible near the 100% loading level.
In addition to 100% loading, it is desirable to keep a constant charging voltage (UDC) on the DC bus 24 (see FIG. 3) when the electric machine operates as a generator. A constant charging voltage provides continuous output to the load and properly returns charge to the battery 26. Assuming that the design point of the ISAD is point DT, as discussed in the preceding paragraph, the phase voltage at idle speed will be significantly less (e.g., 50%) than the phase voltage required for 100% loading. The torque segment DTFT in FIG. 4(a) corresponds to the voltage segment DuFu in FIG. 4(b). Of course, it is difficult to keep a constant charging voltage (UDC) on the DC bus 24 of FIG. 3 if the AC voltage of the generator output varies along curve DuFu from idle speed nidle to a boosted speed nboost, especially when there is a large speed difference between the boosted speed and the idle speed. As mentioned previously, it is preferable to keep the AC line-to-line voltage constant, similar to the segment EuFu in FIG. 4(b). However, achieving constant voltage requires performance of flux weakening within the speed variation range. Flux weakening is not desirable in this situation because it results in a smaller power factor and decreased efficiency of the machine. Therefore, constant voltage may not be practical. The requirement for flux weakening to keep a constant AC line-to-line voltage is shown with respect to equations 1-4. In particular, equations 3 and 4 show that AC voltage is directly related to the back EMF, Eg. Furthermore, equation 1 shows that only flux and frequency (i.e., speed, per equation 2) are variable with respect to the back EMF, Eg. Thus if frequency increases, flux must decrease to keep the per phase back EMF constant.
As shown in FIG. 4(b), once the speed of the electric machine reaches a boosted speed, nboost, full electrical output is produced by the electric machine. Thereafter, the electric machine is controlled so its output power remains constant at this level. This control may be achieved by controlling the flux of the machine and/or by controlling the output of the inverter/rectifier. This speed range is called the constant power region or the flux weakening region. Controlling the electrical output prevents the electric machine from providing to much current and damaging the sensitive instruments associated with the load.
Accordingly, there is a need in the art for an electric machine that is capable of providing increased torque at starting, increased phase voltage near engine idle speed, and constant 100% loading near boosted engine speed. It would be preferable to provide such a machine without requiring significant increases in the size of the electric machine and without sacrificing the efficiency of the machine.